/*
 * Copyright 2000-2015 JetBrains s.r.o.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.intellij.util.containers;

import java.util.Iterator;
import java.util.NoSuchElementException;

/**
 * An iterator with additional ability to {@link #peek()} the current element without moving the cursor.
 */
public interface PeekableIterator<T> extends Iterator<T> {
    /**
     * @return the current element.
     * Upon iterator creation should return the first element.
     * After {@link #hasNext()} returned false might throw {@link NoSuchElementException}.
     */
    T peek() throws NoSuchElementException;

    PeekableIterator EMPTY = new PeekableIterator() {
        @Override
        public Object peek() {
            throw new NoSuchElementException();
        }

        @Override
        public boolean hasNext() {
            return false;
        }

        @Override
        public Object next() {
            return null;
        }

        @Override
        public void remove() {
            throw new UnsupportedOperationException("remove");
        }
    };
}
